This invention relates to a coating for, and method of coating, heat exchanger coils.
Heat exchangers are typically metal coils that conduct heat from one fluid to another fluid, such as from water or Freon (chlorofluorocarbon) inside the coils to air surrounding the coils. Examples of such coils are found in air conditioners, heaters, refrigerators, and dehumidifiers. These coils are usually manufactured from a metal such as copper, iron, brass, or aluminum, or from a metal alloy that contains zinc, manganese, silicon, chromium, nickel, magnesium or carbon. Metals are the preferred materials for heat exchangers because of their high heat transfer coefficient. Aluminum heat exchangers are widely used for car air conditioners because of their light weight, and the fact that aluminum is more efficient than steel as a heat exchange material.
Although metal heat exchanger coils are preferred for their heat transmission properties, they develop at least three very damaging conditions over time with use and exposure to the environment. First, the exterior surface of metal coils corrode on exposure to moisture and other airborne chemicals. Moisture in the air condenses on cooling coils, causing a buildup of water on the surface. The moisture corrodes the coils, causing them to be etched and pitted. This reduces the strength of the coil, and shortens the useful life and efficiency of the unit. This is especially true in areas that have a high salt content in the air.
Second, heat exchanging metal coils experience "fouling," which is the accumulation of dust and other particulate matter on the surface. Fouling decreases the efficiency of heat transfer because the particulate matter lining the outer surface of the coil generally has low thermal conductivity. In addition, fouling contributes a bad odor to the surroundings.
Perhaps the most important problem associated with the use of metal heat exchanging coils is the buildup of bacterial and fungal growth on the surface of the coils, These organisms tend to accumulate and propagate on the surface because of the presence of the moisture and particulate matter. In particular, fungi such as Aspergillus niger, Aspergillus flavus, and Pencillin funiculogum, and bacteria such as Staphylococcus aureus (Gram positive) and Pseudomonas aeroginosa (Gram negative) are known to grow under these conditions. These organisms produce a foul odor in the environment and exacerbate allergy problems. In addition, the organisms have a low thermal conductivity, which decreases the efficiency of heat transfer.
Bacterial and fungal growth are an especially significant problem for automobile air conditioners and heating units. For example, when a car air conditioner is turned on, a fan forces air past the surface of the heat exchanger coils, blowing the particulate and bacterial buildup into the passenger area. Often a strong, foul, musty odor is detected. Not only is the smell unpleasant, but it is unhealthy as well. The debris and organisms cause allergic reactions such as swollen, teary eyes, runny noses, sore throats and asthma.
Likewise, in refrigerators and freezers, a fan forces air past cooling coils and into the food compartments. The cooled air carries organisms that may contaminate the food. Furthermore, when the cooling is stopped, for example, when the refrigerator is unplugged, the organisms, especially mold, proliferate.
It is clear that the three above-described problems associated with the use of metal heat exchanger coils, corrosion, fouling, and biocidal buildup, exacerbate each other. Fouling buildup occurs more rapidly when there is corrosion on the coils, and organisms proliferate at a faster rate when there is fouling particulate and moisture to feed on.
There is therefore a strong, long felt need to develop a coating for metal heat exchangers that reduces the corrosion, fouling and biocidal buildup on the exterior surface of the coils. The desired coating must be durable, efficient, and capable of being applied to coils during manufacture.
Therefore, it is an object of the present invention to provide a coating for heat exchanger coils that protects the exterior surface of the coils from corrosion caused by moisture and other chemicals.
It is another object of the present invention to provide a coating for heat exchanger coils that prevents the buildup of dust and particulate matter on the surface of the coil.
It is a further object of the present invention to provide a coating for heat exchanger coils that prevents the buildup of micro-organisms on the surface of the coil.
It is still another object of the present invention to provide a coating for heat exchanger coils that is suitable for large scale manufacturing.
It is a still further object of the present invention to provide a process to apply a biocidal protective coating to heat exchanger coils that is simple and efficient.